Study session may see lots of fish encountered rising across river (6,4)
I believe the answer is:
school term
'study session' is the definition.
(period of study at school)
'lots of fish encountered rising across river' is the wordplay.
'lots of fish' becomes 'school' (school can mean a group of fish).
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'rising' says the letters should be written in reverse (going up in a down clue).
'across' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'met' backwards is 'tem'.
'tem' enclosing 'r' is 'term'.
'school'+'term'='SCHOOL-TERM'
'may see' is the link.
(Other definitions for school term that I've seen before include "Summer, perhaps" , "when one was younger and in form" .)